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The Weather Network
Dec. 3, 2021 | Friday
Editorials and Opinions
Growing Together: Moth infestations may be targeting your boxwood shrubs
Box tree moths present a threat to boxwoods. (Supplied)

Well, there is a new pest in town and we are just starting to see the damage that it is capable of causing.

Let me introduce you to the box tree moth (Cydalima perspectalis).

As the name indicates, it is primarily found on all species and cultivars of boxwood plants, although there have been reports of them on pachysandra as well.

The box tree moth is native to eastern Asia – Japan, China, Taiwan, Korea, Far East Russian and India. It was first reported in Europe in 2007 and made its Canadian debut in 2018 with the first report of it in the Etobicoke area.

Last summer came the initial report of the moth being in the Niagara-on-the-Lake area. This summer it is becoming more apparent, by the damage that we are seeing, that the box tree moth numbers are quickly increasing in Niagara.

Just last week, I was asked to visit a home in town to check on some boxwoods that were browning and came upon the moth itself – which confirmed that what we were seeing was, indeed, damage caused by the box tree moth.

So, what does the moth look like? The adult moth is medium in size, with a wingspan of about 4 centimetres.

It has white wings with thick, dark brown borders around the entire wing. The body of the moth is also white, with a few dark brown specks on it.

The larva form is a caterpillar that can reach up to 4 centimetres in length. When the larvae first hatch, they are greenish/yellow in colour with a shiny black head. As the larvae mature, they become more green in colour with thick black and thin white stripes along the length of the caterpillar.

The larvae is what causes the damage on the boxwoods.

So, what is the life cycle of the box tree moth?

Here in Canada, there can be two or three generations per year. The adult moth only lives for about two weeks, but before they die, they lay eggs on the underside of the boxwood leaves in clusters of 10 to 12.

The eggs hatch in three days and the young larvae feed on the underside of the boxwood leaves and start to form webs between the leaves.

As the larvae mature, they become skeletonizers leaving just the midribs of the leaf. If there is a heavy infestation it can cause the entire boxwood to brown up in a short time.

It takes about 14 days for the larvae to mature and pupate. Pupae live about 14 days before becoming adults. The box tree moth overwinters in the larval stage, by spinning a silky cocoon between the leaves.

How can you protect your boxwoods?

Routinely examine your boxwoods for signs of chewed leaves, caterpillars, webbing and frass (fancy term for caterpillar poop). The best times to check for larvae are: May 15 to June 15, July 15 to Aug. 15 and Sept. 1 to 20.

If you have the pest, there is a biological control spray called BTK (Bacillus thuringiensis var. kurstaki). The active ingredient of the spray is safe to humans and pets and is sold at most retail garden centres.

BTK application is most effective when used two or three times a year during the time frames listed above. Recheck your plants five to seven days after spraying to see if the mixture has worked.

If you are still seeing caterpillars feeding, repeat the application of BTK. You can also hire a professional to spray for you.

Please note that even though high populations of box tree moth caterpillars may defoliate boxwood plants, and in some cases, result in plant dieback, as long as the outer stems are green, there is a possibility that the shrub can push out new leaves.

In severe cases, though, it may be necessary to remove or trim the infested shrub. Effective removal and disposal of infested plants is crucial to slowing the spread of box tree moth.

Place all cuttings into a black plastic bag, tie it tightly and place in the full sun for 48 hours. This should be enough to kill off the caterpillars. If handpicking the caterpillars, drop them into a bucket of soapy water.

All findings of box tree moth should be reported to officials who are tracking the pest.

Contact the Canadian Food Inspection Agency using the link inspection.gc.ca/about-cfia/contact-us/contact-cfia-online/eng/1299860523723/1299860643049

Joanne Young is a Niagara-on-the-Lake garden expert and coach. See her website at joanneyoung.ca.

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